Method of testing optical materials by irradiating with high energy density radiation, optical materials selected by said method and uses thereof

ABSTRACT

An optical material for lithographic applications is selected from crystal materials by a testing method. The crystal materials are preferably quartz and/or alkali or alkaline earth halides, especially fluorides, or mixed crystals. The testing method includes three tests to measure irreversible radiation damage: 1) the optical material is irradiated with ultraviolet radiation at 193 nm and the non-intrinsic fluorescence intensity at 740 nm is measured; 2) the optical material is irradiated with high energy density laser light and a change in respective absorptions before and after irradiation at 385 nm is measured; and 3) the optical material is irradiated with an X-ray or radioactive source to form all possible color centers and a difference of respective surface integrals of corresponding absorption spectra in ultraviolet spectral and/or visible spectral regions is measured before and after irradiation.

CROSS-REFERENCE

This is a divisional of U.S. patent application Ser. No. 11/440,925,filed on May 25, 2006 now U.S. Pat. No. 7,522,270. In accordance with 35U.S.C. 120 the invention described and claimed herein below is entitledto the benefit of the filing date of the aforesaid U.S. patentapplication, whose disclosure is expressly incorporated by referencethereto. The invention claimed and described herein below is alsodescribed in German Patent Document DE 10 2005 024 678.8, filed May 30,2005.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a method of testing optical materialsby irradiating with high energy density radiation to determine long-termstability of their transmission properties, to the optical materialsobtained with this testing method, and to uses of the optical materialsobtained by this testing method.

2. The Related Art

Electronic computer components such as computer chips and otherintegrated circuits are manufactured by optical lithography. Duringtheir manufacture these circuit structures are imaged by a photomask ona support provided with a photo-lacquer, a so-called wafer, and thecircuits and/or entire electronic devices including them are produced byirradiation. Since the requirements for computer performance are alwaysincreasing, increasingly smaller circuits are required. Because of thatthe respective circuit structures must be imaged ever more sharply, i.e.with greater resolution, which leads to the use of ever smallerwavelengths for irradiation of the photo-lacquer. However radiation witha shorter wavelength has a correspondingly higher energy.

It is also known that materials for optical elements absorb radiationpassing through them so that the intensity of the radiation is generallyless after passing through those materials than its initial valueimmediately prior to entering the materials. Moreover additionalabsorption and scattering effects occur at the surfaces though which theradiation passes, which similarly leads to a reduction of thetransmission of the radiation. It is also known that the amount ofabsorption depends not only on the wavelength of the radiation, but alsoon the energy density or the fluence. When the path length of a lightbeam through the entire optical material of a lens system can be longerthan a meter without more during irradiation of these smaller chipstructures or circuits, absorption of the radiation passing through thelens system is a great problem. For these optical systems it is thusdesired that the absorption is kept as small as possible, i.e. thesesystems and their elements should have a high permeability ortransmission at least for the respective working wavelengths that areused in the system. It is also known that the absorption comprisesmaterial-specific (intrinsic) parts and so-called non-intrinsic parts,which result from inclusions, impurities and/or crystal defects.Intrinsic absorption is constant and depends on the nature of thematerial. It is independent of the quality of the material and thus doesnot decrease. The additional non-intrinsic absorption depends on thequality of the material, i.e. depends on the extent of theabove-mentioned impurities, crystal defects, etc and thus can beavoided, at least theoretically. It leads to quality losses in theoptical material and thus in the lens system.

Energy, which leads to heating of the material, is deposited in theoptical material by intrinsic and non-intrinsic absorption. This sort ofheating of the material has the disadvantage that optical properties,such as the index of refraction change, since the index of refractiondepends not only on the wavelength of the light but also on thetemperature of the optical material, which leads to changes in theimaging behavior in an optical component used for beam formation.Furthermore heating of an optical component leads to thermal expansionand thus to a change of the lens geometry. These phenomenon produce achange of the lens focal point and to some extent blur the projectedimage formed by a heated lens. In photolithography, as it is used formaking computer chips and electronic circuits, this causes a decrease inquality and an increase in waste and thus is not desired.

Furthermore in many materials a portion of the absorbed radiation notonly is converted into heat but also into fluorescence, which similarlyis produced by impurities and crystal defects.

Attempts have thus already been made to determine the optical quality ofthese materials prior to their processing into optical elements. Thus WO2004/027 395 describes a method for determining the properties of anoptical material used for making optical elements, in which aradiation-induced absorption is measured in an optical material byirradiating it with an exciting radiation and measuring the totalfluorescence comprising the intrinsic portion induced by this excitingradiation and the non-intrinsic portion. The non-intrinsic portion ofthe fluorescence is determined during and/or immediately after theirradiation.

In German Patent Document DE 103 35 457.3 A1 a method is described forquantitative determination of properties of the crystals used foroptical elements at high energy densities, in which theradiation-dependent transmission at wavelengths in the ultraviolet (UV)is determined by radiation-induced fluorescence. In this method at leastone induced fluorescence intensity maximum is determined by measuringnonlinear absorption processes at various fluences (H), determining theslope of the transmission curve from that determination, |dT/dH|, andthe transmission from this slope. The so-called rapid damage process RDPmay be established with this method.

In German Patent Document DE 100 50 349 A1 a method for determining theradiation stability of crystals is described. In this method the changeof the absorption coefficient is measured before and after irradiation.In a first measurement the absorption spectrum A of a crystal, or of apiece of the crystal split off or cleaved from it, is measured over apredetermined wavelength range from λ₁ to λ₂ by means of aspectrophotometer. Then the crystal or cleaved piece of it is irradiatedwith an energetic radiation source for forming all theoreticallypossible color centers. After the irradiation the absorption spectrum Bof the crystal or cleaved piece of it is measured in a second absorptionmeasurement over the same wavelength range from λ₁ to λ₂. Subsequentlythe surface integral of the difference spectrum formed from theabsorption spectra A and B over the range of wavelengths from λ₁ to λ₂is formed and divided by the thickness D of the crystal. The absorptioncoefficient Δk induced by the working wavelength used in laterapplications is determined from this result.

European Patent Document EP 0 875 778 A1 states that the absorption of aCaF₂ crystal is essentially caused by sodium impurities, which aretypically in a range of about 0.1 ppm, in an image focusing opticalsystem for a UV laser. According to that the other possible impurities,such as strontium, etc, contribute to the production of non-intrinsicabsorption to an essentially small extent.

In European Patent Document EP 0 875 778 A1 a material to be tested isirradiated with an energetic ArF laser with a frequency of severalhundred Hz for several seconds or minutes and the absorption spectrumprior to or after irradiation is determined. The irradiated energy perpulse amounts to 1 μJ to several Joule per pulse with a pulse durationof 10 to 20 ns. It was established that the absorption produced onirradiation of quartz glass and CaF₂ with a laser does not correspondwith that, which is measured by the weak light beam of aspectrophotometer. Thus it was found that the permeability ortransmission of the material at the start of irradiation dropscomparatively rapidly until after about 10⁴ pulses and after thatremains constant. Moreover it was expressly established thatsubsequently the transmission does not change further, so that theabsorption is determined after about 10⁴ to 10⁵ pulses.

However all these methods only determine short-time, reversibleradiation damage. This short-time reversible radiation damage isreversible by further irradiation or heat treatment, which means thatthe radiation damaged structures again relax. Up to now it was thoughtthat the irreversible radiation damage that is known to occur in quartzglass, which causes a slow and irreversible increase in absorptionduring long-term use of this optical material over several years, didnot occur in crystals. In the meantime however long-duration testsestablished that irreversible permanent damage also occurs in crystalsafter 10⁸ to 10⁹ laser pulses at energy densities of from 5 to 25 mJ/cm²over a period of several weeks.

The current procedure for determining this permanent damage comprisesdetermining the transmission T and/or the absorption A per input energydensity or fluence H for the optical material measured and from thatdetermining the slope of this curve |dT/dH| and/or |dA/dH|. The amountof absolute transmission or the initial absorption at an input energydensity H=0 may then be ascertained from this curve by extrapolation to0. This value normalized to the sample thickness is characterized as theinitial absorption k₀. Subsequently the optical material is irradiatedwith a higher energy density of about 1 Giga pulse with 10-12 mJ/cm² andafter this irradiation as described previously the initial absorptionand/or absolute transmission is determined. The difference of therespective determined initial absorptions k₀ (or absolute transmissions)before and after irradiation is a reliable measure for the long-termstability of the optical material. Optical materials with Δk₀ values of>4×10⁻⁴ cm⁻¹ have proven to be unusable.

Since the service life of this sort of lens system in steppers oftenamounts to ten years and more, a statement regarding the irreversibleradiation damage that would occur over time is already required andunsuitable materials must be sorted out. Thus not only considerablycosts for expensive manufacture of optical lens systems, but alsoillumination errors are avoided, whereby the yields during chipmanufacture are increased. A simple determination of these long-termstabilities, which as much as possible is performable in a few hours,has currently not been possible and currently only occurs by theabove-described pulsed laser shot method that takes several weeks.

The determination of long-term absorption increase in an endurance orstability test performed in a comparative short time has not beenpossible for practical reasons. Up to now no possible procedure has beenfound, with which a statement regarding the change of optical materialproperties over the entire service life can be obtained fromexperimental results obtained over a short time interval.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a testing method ofthe above-described kind with which irreversible or permanent absorptionchanges of optical materials caused by radiation damage occurring duringlong-term usage of the materials can be quickly determined in a shorttime interval, in order to select suitable optical materials, especiallyfor lithographic applications, in this manner.

It is another object of the present invention to select suitable opticalmaterials for long-term usage, especially for lithographic applications,with the testing method according to the present invention.

It is a further object of the present invention to provide opticalcomponents, especially for lithographic applications, that are made withthe optical materials that are determined to be suitable with thetesting method according to the present invention.

These objects and others, which will be made more apparent hereinafter,are attained in a method of determining the extent of irreversibleradiation damage of an optical material, which has a small tendency toexperience such permanent radiation damage and which has a constantabsorption during long-term irradiation with ultraviolet radiation of ahigh energy density.

According to the invention three tests are performed in order todetermine the radiation damage. These three tests comprise the steps of:

a) irradiating an optical material to be determined with ultravioletradiation at a wavelength below 250 nm, preferably of 193 nm, anddetermining the non-intrinsic fluorescence intensity at a wavelength of740 nm;

b) irradiating the optical material with laser light of a high energydensity and measuring a change in the respective absorptions at awavelength of 385 nm before and after irradiation; and

c) irradiating the optical material with an energetic radiation sourceso as to form all possible color centers and measuring a difference ofrespective surface integrals of the corresponding absorption spectra inultraviolet spectral regions and/or visible spectral regions before andafter irradiation.

The combination of these three special test procedures, which allindicate only the short-term immediately occurring reversible radiationdamage, surprisingly may also establish whether this optical materialalso develops irreversible radiation damage during long-term Irradiationand thus whether its absorption irreversibly increases during longduration usage. This is all the more surprising since the testprocedures used here only directly indicates the rapid short-termrapidly arising reversible radiation damage, the so-called rapid damageprocess (RDP), which is based on a completely different mechanism thanthat occurring in irreversible radiation damage, which occurs duringlong-term usage. Only when a material fulfills predetermined setstandards in all three tests is it guaranteed that no great or damagingirreversible absorption changes take place during long duration exposureto energetic radiation. For the testing procedure according to theinvention the order in which the three test steps are performed is notimportant.

In the first test the non-intrinsic fluorescence is measured alreadyduring excitation with light and/or immediately after termination of alight pulse, i.e. after the light pulse as passed through the sample.

In a preferred embodiment of the invention in addition to thenon-intrinsic fluorescence the intrinsic fluorescence is measured forstandardization. Since the intrinsic fluorescence is a material constantthe non-intrinsic fluorescence can be standardized from the size ratioof the non-intrinsic to intrinsic fluorescence and thus it can berapidly determined whether the tested optical material is suitable forfurther processing to an optical element, such as a lens, prism, etc.Furthermore the amount of the respective impurities can be determinedfrom this ratio with the aid of a simple calibration curve. Otherwise anexpensive calibration, e.g. dimension determination, etc are required inthe sample to be tested.

The procedure for performance of the method according to the inventionincludes determination of the fluorescence at a wavelength of 740 nm.This fluorescence band has proven to be especially sensitive in themethod according to the invention. Preferably an intrinsic fluorescenceband is used to normalize the non-intrinsic fluorescence. The intrinsicfluorescence band at 278 nm is especially suitable for standardizationor normalization. The normalization of the height of the measurednon-intrinsic fluorescence band or bands occurs by formation of theratio of the non-intrinsic to intrinsic fluorescence (intensity). Thenon-intrinsic fluorescence is measured synchronized to the input laserpulses in an entirely preferred embodiment of the method according tothe invention. A ratio of less than 1:100, preferably 1:200, and mostpreferably 1:500, shows that this optical material has passed the firsttest.

It has proven to be especially suitable to treat the material to betested by means of a pre-irradiation prior to performing the methodaccording to the invention. The absorption state of the materialregarding the so-called rapid damage and/or rapid annealing is saturatedwith a predetermined number of laser pulses, so that the measurementsfollowing that occur from a constant base line.

A typical number of laser pulses during pre-irradiation is at least3000, preferably at least 6000 and 30,000, preferably 70,000 to 200,000.Principally it is required for performing the method according to theinvention that the material is irradiated with as comparable as possibleenergy densities in the material to be tested. Preferably the method isperformed not only with the same or comparative energy densities, butpreferably with equal excitation and fluorescence wavelengths in orderto obtain comparative values, especially spectra.

Finally the sort of impurities even in amounts in, the ppb range can bedetermined without difficulties from the fluorescence spectra. Theamount of the impurities producing the fluorescence spectra isdeterminable from the ratio of the intrinsic to non-intrinsicfluorescence. The impurity materials are usually rare earths andespecially cerium, europium, terbium, sodium and oxygen from oxides.

According to the invention it is preferred to perform the irradiation inthe first test step with UV light and especially far UV light. UV lightwith wavelengths below 250 nm, especially below 200 nm has proven to besuitable. However UV light with wavelengths between 100 or 150 nm and200 nm is especially preferred. Appropriately the method is performedwith those excitation wavelengths, with which the optical materialshould be irradiated during later applications. A preferred radiationsource for the energetic light is a laser that produces laser pulses,preferably with working wavelengths of 193 nm.

In the method according to the invention the non-intrinsic fluorescenceis preferably measured by means of a grating spectrograph and an I-CCDcamera with adjustable illumination intervals (intensified chargecoupled device). Preferably the obtained spectrum is processed undercomputer control. This type of measurement and apparatus are well knownto those skilled in the art and are described, for example, by W.Triebel, et al, in Proceedings SPIE, Vol. 4103, pp. 1 to 11, (2000),“Evaluation of Fused Silica for DUV Laser Applications by Short TimeDiagnostics”, or also by Mizuguchi, et al, in J. Opt. Soc. Am. B, Vol.16, 1153 and following (July 1999).

According to the invention a blocking device, which prevents the passageof the excitation radiation, is preferably arranged between thefluorescing sample to be tested and the fluorescence measuring device.This sort of blocking device, which can mask arbitrary excitationwavelengths, is known to those skilled in the art. The blocking canoccur in many different ways. On possibility is to block radiation atthese wavelengths by means of a grating spectrograph arranged in frontof a CCD camera, which divides the received light into its differentwavelengths. It is possible to block out or deflect the excitationradiation from an energetic radiation source by suitable arrangement orrotation of the spectrograph. Principally it is also possible to rotatethe grating spectrograph of the CCD camera.

An additional possibility consists of the use of wavelength specificfilters, such as dielectric thin film filters, which are currentlyselectively made for wavelengths of choice. This sort of filter isusually made by applying multiple reflecting layers on a supportmaterial, which prevents the passage of the undesired wavelengths.

Such layer filers are preferred in the method according to theinvention. However it is necessary that the filter used have noself-fluorescence produced by the incident light, so that falsemeasurement results are not produced.

The determination of the fluorescence according to the invention occursespecially within or immediately after an irradiation time interval forthe material. It preferably occurs within a time interval at the end ofor after the irradiation of the material, which corresponds to therespective characteristic decay curves or lifetimes of the variousnon-intrinsic fluorescence emissions, or is adjusted to them. In aplurality of cases 90%, especially 80%, often also 70% of the decay timehas proven suitable for the measurement. In a few preferred opticalmaterials the method according to the invention and the determination isperformed within a time interval of less than 50 nsec after the end ofthe irradiation or the irradiation pulses in the material. However it ispreferred to perform the determination within a time interval of at most40 nsec after the end of the irradiation, but at most 30 nsec isespecially preferred. In a few cases it has proven to be appropriate tocomplete the measurement within a time interval of less than 15 nsec,after the irradiation of the material has been performed.

The CCD cameras preferred for use in the method according to theinvention are the so-called OMAs (Optical Multichannel Analyzer orintensive Optical Multichannel Analyzer), especially with adjustableillumination or measurement intervals. One such camera has a detectionlimit of less than 10 photons and permits a small illumination time forexample 10 nsec or even down to 150 psec. This sort of camera iscommercially obtainable, for example, from Roper Scientific, USA, amongother sources.

The second test is performed so that the material to be tested isirradiated with high energy densities of at least 1 mJ/cm², especiallyat least 5 mJ/cm². Preferably the minimum energy density amounts to 25mJ/cm², especially 50 mJ/cm². In principle, the energy density does nothave an upper limit. However a maximum energy density of 200 mJ/cm²,particularly 150 mJ/cm², is suitable. However a maximum energy densityof 120 mJ/cm², especially of 100 mJ/cm², is preferred. The radiation ispreferably performed with a laser. An appropriate laser is a laser witha wavelength of 193 nm, e.g. an ArF laser. The energetic irradiation issuitably performed by irradiation with at least 10⁴, preferably at least2×10⁴ and especially preferably at least 3×10⁴ pulses. A minimum numberof 4×10⁴ or 5×10⁴ is especially preferred. Also an upper limit accordingto the invention is not relevant in the case of the number of pulses.However for practical reasons an upper limit of 20×10⁴, especially15×10⁴ or 10×10⁴ has proven to be suitable. Subsequently the absorptionspectrum is taken in the irradiated spot and the difference in theabsorption spectra before and after irradiation is determined. For thispurpose the absorption spectra must be taken with radiation having aslittle incident energy as possible, so that the rapid radiation damageproduce, i.e. the rapid damage process, does not relax again. If thedifferences between both of these absorption spectra exceed certainpredetermined set values, then the tested material does not have therequired long-term stability.

The absorption spectra are preferably taken in the ultraviolet andvisible region, which means in a range between 190 nm and 800 nm.Understandably only one or a few intervals in this range are testedaccording to the invention, or even only one band is tested. Theabsorption band at 380 nm is especially preferred for quality controlaccording to the invention. To achieve the required long-term stabilityfor the absorption behavior the change of the absorption in this bandshould amount to less than 2.5 or 2×10⁻³ cm⁻¹, preferably less than1×10⁻³ cm⁻¹ and especially preferably less than 0.5×10⁻³ cm⁻¹.

In the third test for determining according to the invention whether amaterial to be tested has the required long-term transmission stabilityfor use in photolithography the absorption at a peak in the spectra at awavelength of 265 nm is measured. Then the optical material is excited,preferably with a short wavelength radiation and of course preferably aslong as all or nearly all theoretically possible color centers have beenformed and after that the absorption is measured again at the samewavelength as before the irradiation. It has been shown that long-termstability is then obtained when the difference of the absorption is lessthan 50×10⁻³ cm⁻¹, but a difference of less than 30×10⁻³, especially20×10⁻³, is preferred. However differences less than 10×10⁻³ andespecially less than 5 or 3×10⁻³ are most preferred.

The radiation damage in conventional methods is preferably produced withthose wavelengths, which would be used in the later optical elements.X-ray sources and other energetic sources are suitable radiation sourcesfor performing the induced absorption according to the presentinvention. For example, radioactive Co⁶⁰ is preferred as a source ofradiation because it is economical and easy to handle and readilyavailable, but the X-ray sources are especially preferred.

The energy density required for performing the method of the inventionis widely variable and depends only on the time interval in whichsaturation should be achieved. However usually energy densities of10³-10⁵ Gy, preferably 5×10³-5×10⁴ Gy (1 Gy=1 J/kg), are employed. Thiscorresponds to doses of a few 10s of J/cm², at the conditions describedherein below. In comparison to test 2 that over 90% of the energy is notabsorbed during irradiation with a 193 nm laser beam, while a muchgreater percentage of the energetic radiation is absorbed in the case ofX-rays must be considered. The irradiation time until saturation occurstypically amounts to 10 to 360 minutes, preferably 30 to 180 minutes. Tocontrol the saturation according to the invention a second irradiationof the sample can be performed and the intensities of the respectiveabsorption bands or absorption spectra can be compared with each other.If no change is observed in the intensities of the absorptions, thedesired saturation was reached with the radiation.

In order to guarantee that all color centers are actually excited in theoptical material, the thickness of the irradiated sample or piece shouldnot be too large, because with greater sample thickness according to theresistance of the sample to radiation uniform permeation or penetrationof the entire sample with the radiation cannot be guaranteed. Also it isnot possible to guarantee that the largest portion of the incidentradiation is already absorbed where possible in the first part of theirradiated thickness. This would lead to different formation of colorcenters with distance from the sample surface, through which theincident radiation passes into the sample.

Radiation conditions should be selected at which all color centers areexcited or formed. If the spectrum after irradiation is now comparedwith the spectrum prior to irradiation, their difference gives directlythe saturation condition and shows the absorption with maximum intensityin the selected wavelength range, which can be produced with the laterused working wavelength during irradiation with at that wavelength.

A great advantage of this test is that the sample or cleaved piece ofcrystal neither needs to be polished nor does its thickness need to beprecisely adjusted. Thus any cleaved piece of the crystal can be used asthe sample. Since crystals usually cleave along their crystal axes,parallel surfaces are always present, which are available formeasurement of the absorption spectra with a spectrophotometer. Thespacing of the surfaces from each other, i.e. the thickness of thecrystal or the path length of the light in the crystal can beconventionally determined by means of a sliding caliper or micrometerscrew. The light beam of the spectrophotometer preferably penetrates thecrystal perpendicular to the crystal surface in order to determine theabsorption or the radiation damage.

A difference spectrum, with which the resistance of the crystal toradiation is determined, is produced by measuring the amount ofabsorption before and after irradiation. The maximum absorptioncoefficient Δk [1/cm] can be calculated according to the Lambert-Beerlaw without more using the known distance that the radiation passesthrough the cleaved piece of crystal or sample. Preferred workingwavelengths are those of lasers, especially excimer lasers, such as theArF excimer laser, also 193 nm.

Preferred obtained materials include quartz and/or crystal materials.Alkali or alkaline earth halides, especially the fluorides, arepreferred crystal materials. CaF₂, BaF₂, MgF₂, SrF₂, LiF, KF, and NaFare most preferred. Mixed crystals of the form KMgF₃ are also especiallypreferred. However principally other known mixed crystals, which areproduced by doping a base crystal with suitable doping materials, arealso suitable. Likewise cubic perovskite, cubic garnet, cubic spinel,and cubic M(II) and M(IV) oxides are suitable. Examples of the cubicgarnet are of the following formulae: Y₃Al₅O₁₂, Lu₃Al₅O₁₂, Ca₃Al₂Si₃O₁₂,K₂NaAlF₆, K₂NaScF₆, K₂LiAlF₆, and Na₃Al₂Li₃F₁₂. MgAl₂O₄, (Mg,Zn)Al₂O₄,CaAl₂O₄, CaB₂O₄ and LiAl₅O₈ are examples of cubic spinel. BaZrO₃ andCaCeO₃ are examples of cubic perovskite. MgO and (Mg,Zn)O are examplesof cubic II/IV-oxides.

The invention also includes the optical materials obtained using themethod according to the invention and their usage in optical imagingsystems. It also includes steppers, lasers especially excimer lasers,computer chips, as well as integrated circuits and electronic devices,which contain these circuits and chips, which contain optical materialsthat are selected or obtained using the testing method according to theinvention.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following examples, withreference to the accompanying figures in which:

FIG. 1A shows two fluorescence spectra for crystal samples 1 and 2 takenin the first testing step of the method according to the presentinvention;

FIG. 1B is a graphical illustration showing the respective behaviors ofabsorbance at 193 nm versus fluence for the two crystal samples 1 and 2;

FIG. 2A is a graphical illustration showing two difference spectra forcrystals 3 and 4 measured in the second testing step of the methodaccording to the present invention;

FIG. 2B is a graphical illustration showing two difference spectra forcrystals 3 and 4 measured in the third testing step of the methodaccording to the present invention;

FIG. 3A is a graphical illustration showing two difference spectra forcrystals 5 and 6 measured in the second HE testing step of the methodaccording to the present invention; and

FIG. 3B is a graphical illustration showing two difference spectra forcrystals 5 and 6 measured in the third X-ray irradiation testing step ofthe method according to the present invention;

EXAMPLES Example 1 Test 1—Irradiation with a Laser Beam at 193 nm andMeasurement of Fluorescence at 740 nm

The changes of the initial absorption at 0 energy Δk₀ at 193 nm weremeasured in six crystal samples that were grown differently and that hada length of 10 mm. The respective transmission values for each samplewere measured at different energy densities and the absorptions at anenergy density 0 mJ/cm² were obtained by extrapolation of these measuredtransmission values. Subsequently they were irradiated with an ArF laserwith a working wavelength of 193 nm with about 1 Giga-pulse and anenergy density of 10 to 12 mJ/cm². The initial absorption was determinedonce again for each crystal sample after irradiation by the same method.The change is given as Δk₀.

Prior to irradiation the non-intrinsic fluorescence at 740 nm wasdetermined in crystals 1 and 2 according to the test analogous to thatdescribed in WO 2004/027 395. The results are given in the added TableI. In this table it is shown that the crystal 1 is characterized by acomparatively intense fluorescence band at 740 nm, while hardly anyfluorescence occurs at this location in crystal 2. The results are shownin FIGS. 1A and 1B.

TABLE I k₀, ABSORPTION AT 0 ENERGY DENSITY & Δk₀ DUE TO IRRADIATION(Test 1) Prior to Irradiation Change due to Irradiation k₀ [10⁻⁴ cm⁻¹]Δk₀ [10⁻⁴ cm⁻¹] Crystal 1 2.5 4.5 Crystal 2 1.2 0.6

From this it is apparent that both crystals have a comparable initialabsorption prior to irradiation. However crystal 1 has a comparativeintense fluorescence band at 740 nm and then crystal 1 is not satisfythe minimum requirements for this sort of optical material according totest 1. After irradiation crystal 1 is characterized by a change of theinitial absorption of 4.5×10⁻⁴ cm⁻¹, while the change of the absorptionof the crystal 2, which has hardly any fluorescence band, only amountsto 0.6×10⁻⁴. This means that a material obtained from crystal 1 forlithographic applications is unusable, since it is not onlycharacterized by a comparatively intense fluorescence band for shortduration uses, but also by a strong increase in long-term damage forlong-term usage and an increase in absorption associated with thatincrease.

Example 2 Measurement and Change of the Spectral Properties due to HighEnergy Irradiation (Test 2) and after Irradiating with X-Rays (XRD)(Test 3)

a) The procedure described here is used for crystals, which weredetermined to be suitable in the LIF Evaluation (Test 1). In other wordsthe crystals tested in tests 2 and 3 are those which had hardly any orno emission bands at 740 nm as measured in test 1. The crystals 3 and 4were tested prior to irradiation for determination of Δk₀ both with thetest procedure 2 and also with test procedure 3. The results aregraphically illustrated in FIGS. 2A and 2B. As shown in FIG. 2A, bothcrystals 3 and 4 appear to be comparable in the procedure according totest 2, however are clearly different as seen from the differencespectra as shown in FIG. 2B, which were obtained according to the testprocedure 3.

After that the initial absorption prior to and after irradiationaccording to the invention was determined for both crystals. The resultsare tabulated in Table 2. The initial absorption and the absorptionchange were standardized or normalized after a long duration irradiationby 10⁹ laser pulses with 10 mJ/cm².

TABLE II k₀, ABSORPTION AT 0 ENERGY DENSITY & Δk₀ DUE TO IRRADIATIONPrior to Irradiation Change due to Irradiation k₀ [10⁻⁴ cm⁻¹] Δk₀ [10⁻⁴cm⁻¹] Crystal 4 1.4 4 Crystal 3 1.1 10

b) It has been shown that the determination of the XRD value accordingto test 3 alone is not sufficient for determination of the long-termlaser stability. For this purpose the HE test 2 (FIG. 3A) and the X-RayIrradiation Test (Test 3) (FIG. 3B) were performed on the crystals 5 and6. Crystal 6 had poorer properties as shown by the HE test 2, althoughcrystal 6 did have only slightly poorer properties in the XRD test 3.The formation of the difference spectra for these crystals is shown inTable 3.

TABLE III k₀, ABSORPTION AT 0 ENERGY DENSITY & Δk₀ DUE TO IRRADIATIONPrior to Irradiation Change due to Irradiation k₀ [10⁻⁴ cm⁻¹] Δk₀ [10⁻⁴cm⁻¹] Crystal 6 4.5 4.5 Crystal 5 0.8 2.4

The table shows that the crystal 6 has a clearly stronger absorptionchange during long-term irradiation than the crystal 5.

This proves that other radiation-induced damage of the optical materialcan be detected by means of the method according to the invention thanwith current prior art methods and indeed that long-termradiation-induced damage that occurs over a much longer usage period offor example ten years and after many hundreds of million laser shots canbe detected by the method of the invention. The procedure according tothe invention may be performed with all three tests over a time intervalof less than a day.

While the invention has been illustrated and described as embodied in amethod of testing optical materials by irradiating with high energydensity radiation, optical materials selected by the method and usesthereof, it is not intended to be limited to the details shown, sincevarious modifications and changes may be made without departing in anyway from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appendedclaims.

1. An optical material for making optical components for lithographic applications, wherein said optical material is selectable by a testing method, said testing method comprising the steps of: a) irradiating an optical material to be tested with ultraviolet radiation at a wavelength below 250 nm, determining intrinsic fluorescence intensity at 285 nm, determining non-intrinsic fluorescence intensity at a wavelength of 740 nm, and subsequently determining a ratio of the non-intrinsic fluorescence intensity to the intrinsic fluorescence intensity; b) irradiating the optical material to be tested with laser light and measuring a difference between respective absorptions at a wavelength of 385 nm before and after irradiation with the laser light; and c) irradiating the optical material to be tested with an energetic radiation source until saturation is achieved and measuring a change in absorption at 265 nm before and after irradiation with said energetic radiation source; wherein said energetic radiation source is an X-ray source or a radioactive source; and wherein said optical material is selectable for making said optical components if the ratio of the non-intrinsic fluorescence intensity at 285 nm to the intrinsic fluorescence intensity at 740 nm is less than 1:100, said difference between said respective absorptions at 385 nm before and after the irradiation with the laser light is less than 2.5×10⁻³ cm⁻¹; and the change in the absorption at 265 nm before and after irradiation with the energetic radiation source is less than 50×10⁻³ cm⁻¹.
 2. The optical material as defined in claim 1, which is an alkali halide crystal material, an alkaline earth halide crystal material, or a mixed crystal material.
 3. The optical material as defined in claim 1, which is a crystal material and is a fluoride selected from the group consisting of CaF₂, BaF₂, MgF₂, SrF₂, LiF, KF and NaF.
 4. The optical material as defined in claim 1, wherein said wavelength of said ultraviolet radiation below 250 nm is 193 nm.
 5. The optical material as defined in claim 1, wherein said determining said non-intrinsic fluorescence intensity is performed within a predetermined time interval after an end of the irradiating at said wavelength below 250 nm and said predetermined time interval corresponds to 80% of a decay time of said fluorescence intensity.
 6. The optical material as defined in claim 1, wherein said laser light has a high energy density of from 25 to 150 mJ/cm² and during the irradiating with said laser light the optical material is irradiated with from 3 to 20×10⁴ laser pulses of said laser light.
 7. The optical material as defined in claim 6, wherein said laser light has a wavelength of 193 nm and is produced by an ArF laser.
 8. The optical material as defined in claim 1, wherein said method further comprises measuring respective surface integrals of corresponding absorption spectra between respective wavelength limits of 240 nm and 300 nm.
 9. The optical material as defined in claim 1, wherein said method additionally comprises the step of pre-irradiating the optical material with at least 3000 laser pulses prior to said irradiating, so that an absorption state of the optical material associated with rapid reversible radiation damage is saturated.
 10. An optical material for making optical components for lithographic applications, wherein said optical material is selectable by a testing method, said testing method comprising the steps of: a) irradiating an optical material to be tested with ultraviolet radiation at an excitation wavelength below 250 nm, determining intrinsic fluorescence intensity at 285 nm and non-intrinsic fluorescence intensity at a wavelength of 740 nm, and subsequently determining a ratio of the non-intrinsic fluorescence intensity to the intrinsic fluorescence intensity; b) if said ratio determined in step a) is less than 1:100, then irradiating the optical material to be tested with laser light and measuring a difference between respective absorptions at a wavelength of 385 nm before and after irradiation with the laser light; c) if said ratio determined in step a) is less than 1:100, then irradiating the optical material with an energetic radiation source until saturation is achieved and measuring a difference of respective surface integrals of corresponding absorption spectra between respective wavelength limits of 240 nm and 300 nm before and after irradiation with said energetic radiation source; and d) establishing that the permanent radiation damage of the optical material is sufficiently small and selecting the optical material for making optical components for lithographic applications from the optical material when said difference between the respective absorptions at the wavelength of 385 nm before and after irradiation with the laser light is <2.5×10⁻³ cm⁻¹ and the difference of the respective surface integrals between the respective wavelength limits of 240 nm and 300 nm before and after irradiation with the energetic radiation source is <50×10⁻³ cm⁻¹.
 11. The optical material as defined in claim 10, which is an alkali halide crystal material, an alkaline earth halide crystal material, or a mixed crystal material.
 12. The optical material as defined in claim 10, which is a crystal material and is a fluoride selected from the group consisting of CaF₂, BaF₂, MgF₂, SrF₂, LiF, KF and NaF.
 13. The optical material as defined in claim 10, wherein said excitation wavelength of said ultraviolet radiation below 250 nm is 193 nm, said determining of said non-intrinsic fluorescence intensity is performed within a predetermined time interval after an end of the irradiating at said excitation wavelength, and said predetermined time interval corresponds to 80% of a decay time of said non-intrinsic fluorescence intensity.
 14. The optical material as defined in claim 10, wherein said laser light is generated by an ArF laser and has a wavelength of 193 nm, said laser light has a high energy density of from 25 to 150 mJ/cm², and during the irradiating with said laser light the optical material is irradiated with from 3 to 20×10⁴ laser pulses of said laser light.
 15. A lens, prism, light conducting rod, optical window, optical component for DUV photolithography, stepper, excimer laser, wafer, computer chip, integrated circuit, or an electronic unit containing said computer chip or said integrated circuit, which are made from or contain optical materials that were selected according to claim
 1. 16. A lens, prism, light conducting rod, optical window, optical component for DUV photolithography, stepper, excimer laser, wafer, computer chip, integrated circuit, or an electronic unit containing said computer chip or said integrated circuit, which are made from or contain optical materials that were selected according to claim
 10. 